Elastic threads or yarns change their length depending on the tension they are subject to, and even very great changes in length are possible. For instance, elastomer yarns are now being used that can be stretched to seven times their original length, or even more. Therefore, the yarn quantity furnished depends greatly on the tension at which the yarn is fed. While in many cases it suffices for the tension of the elastic yarn to be kept constant within specific limits, there are a number of applications in which it is important for a predetermined yarn quantity to be furnished within a predetermined time interval. The yarn quantity is defined for instance by a yarn length at a specified yarn tension. If the yarn is very elastic, it cannot be guaranteed that a certain yarn quantity will be furnished.
While in some applications the yarn demand is constant over time, there are many applications in which the yarn demand fluctuates very greatly over time. Examples are flatbed knitting machines and similar knitting machines, circular knitting machines with pattern equipment, stocking knitting machines, and so forth. A knitted product must often be made on these machines in which the elastic yarn is knitted together with one or more inelastic yarns, so-called hard yarns. This must be done with proportional quantity in many cases; that is, for a given quantity of hard yarn, a corresponding quantity of elastic yarn must be furnished positively and with as much as possible the same speed profile. This is relatively difficult because of the different qualities. It has been found that synchronized quantitative feeding of hard and soft yarns is relatively difficult, at least whenever yarn consumption is fluctuating.
From German Patent Disclosure DE 38 24 03, a yarn feeder for fluctuating yarn demand is known. The yarn feeder has a yarn feed wheel, driven by a stepping motor, whose outer circumference is defined by six wire brackets. A yarn brake is disposed upstream of the yarn feed wheel in terms of the yarn travel. The yarn feed wheel is followed by a yarn tension sensor for regulating the stepping motor and thus the yarn quantity. The yarn feeder serves to feed yarn at a constant tension.
From German Patent Disclosure DE 42 06 607 A1, a yarn feeder is known that is intended to furnish time-varying quantities of elastic yarns. The yarn feeder has a disk rotor motor, which is connected to a yarn feed wheel. The yarn to be furnished wraps around the yarn feed wheel. A fixedly set yarn brake is disposed upstream of the yarn feed wheel and contains a permanent magnet to generate the braking action or an electromagnet for adjustably generating this action. To monitor the yarn tension and keep it constant, the yarn feed wheel is followed by a yarn tension sensor which controls the disk rotor motor.
The yarn feeder serves to feed elastic yarns at a more or less constant tension, but not to positively feed fixed yarn quantities.
From European Patent Disclosure EP 0 499 380 A1, the supply of elastic yarns to a yarn consuming station with constant yarn consumption is known. The yarn feeder device includes two yarn feeders operating independently of one another through which the yarn passes in succession. First, it is guided by a yarn feeder that draws the yarn from bobbins. The yarn feed wheel is driven by an electric motor, which is triggered in accordance with the yarn tension detected by a yarn tension sensor disposed just downstream of the yarn feed wheel. Thus the yarn is fed at more or less constant tension to two yarn feed wheels or rollers in frictional engagement with one another, which are driven at a fixed rpm and thus furnish constant yarn quantities. Yarn quantities that are constant over time are thus carried to a yarn consuming station, in which the elastic yarn is combined with a hard yarn.
This arrangement is not set up to furnish varying yarn quantities over time. A yarn feeder device is also known from Published, Examined German Patent Application DE-AS 158 5111, for furnishing yarns to a knitting machine at a constant yarn speed and at the same time at a constant yarn tension. This yarn feeder apparatus has two uniformly driven conical rollers around which the yarn wraps one after the other. In the process, the yarn passes through one yarn guide for each roller, and the yarn guide defines the diameter at which the yarn rolls onto the roller and off again. By defining the two payout diameters differently, it is possible for the yarn first to be drawn from the bobbin at a small roller diameter and then taken from the next roller with a larger diameter, so that the yarn is greatly stretched between the rollers. In the portion following the second roller, the yarn is relaxed again, utilizing a hysteresis effect of the kinky yarn. This hysteresis effect that can be observed in kinky yarns means that after the second roller, in the relaxed state, the yarn is hardly shortened at all. It should be more easily worked in that state.
This apparatus and the method have been developed especially for processing constant quantities over time of curled yarns with a hysteresis effect.